Project 2. The Role of Iron in Formation of Dioxins and Persistent Free Radicals The previous Project 2, "Development of a Demonstrable Model of Dioxin Formation", was well received by the previous review committee, receiving a priority score of 1.4. It was funded in part for 3 years by NIEHS, the reduced scope of work being concerned with gas-phase and copper-mediated formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F). Although the current project proposal is again concerned with formation and emissions of PCDD/F from combustion and thermal degradation of Superfund wastes, it is now more directly concerned with the themes and issues of this overall Program Project Grant proposal. It specifically focuses on the role of iron contained in fine and ultra-fine particles on the formation of PCDD/F and persistent free radicals (PFRs). In our previous application, we focused on gas-phase reactions of halogenated phenols and surfacemediated reactions of copper because they were known pathways of PCDD/F formation. Copper oxide was known to catalyze the formation of PCDD/F and is typically the second or third most abundant metal in combustion-generated particulate matter. Halogenated phenols were also well-established precursors. Although, iron oxide can be present in 10x the concentration of copper oxide, its role in PCDD/F formation has not been researched. It is unclear why this is the case, especially since we now know, through the generation of our own preliminary data, that it also catalyzes the formation of PCDD/F from chlorinated benzenes (which are 10-100 times the concentration of chlorinated phenols in full scale combustors) as weil as chlorinated phenols. Based on our studies, the yields of PCDD/F from these precursors for iron oxide is equalto or greater-than that observed for copper oxide at identical concentrations and reaction conditions. This clearly indicates that iron can play a greater role in PCDD/F emissions that copper. In addition, our new data indicates that the PCDD to PCDF ratio is lower over iron oxide than copper oxide. This may resolve a longstanding issue that laboratory studies of PCDD/F formation from chlorinated phenols over copper oxide give a PCDD to PCDF ratio of >1, while the PCDD to PCDF ratio from full-scale combustors is 1. This further supports the possibly dominant role of iron in PCDD/F formation. Equally importantly, we also suspect that it promotes the formation and stabilization of halogenated semiquinone- and phenoxyl-type PFRs that are key intermediates in formation of PCDD/F and have been shown to be highly toxic. Free radical damage induced by PFRs is the focus of the three biomedical projects. These projects are study the effects of PFR-copper oxide particles because we have extensive chemical data on the nature of PFRs associated with copper oxide-containing ultrafine particles. This project, along with Project 1, now focus on iron. Thus, we are generating the chemical data necessary for the future study of the toxicology of PFR/iron-containing ultrafine particles In summary, this application focuses on significant new sources of PCDD/F and PFRs involving surfacemediated reactions of iron. We also incorporate newly developed techniques to characterize these PFRs and study the effect of particle size (i.e. ultra-fine (nano) vs fine (micro)). This application still contains the development of detailed reaction kinetic models of surface-mediated reactions which were not included in the previously funded project due to reduction in funding and duration